"In an arc that stretches from Eastern Europe to the Persian Gulf to East Asia, vulnerable states on the outer periphery of U.S. power are re-examining their strategic menus in the face of rising or revisionist powers, most notably China, Russia and Iran," Mitchell and Grygiel state.

These vulnerable nations feel compelled to act as a result of U.S. inaction, they say.

"The frenzy of activity ... shows that all is not well in these regional ecosystems. America's frontline allies are less confident of U.S. strength and fidelity," the duo writes.

"This is partly a result of recent U.S. policy, which has often seemed to downgrade alliances in favor of accommodation with large, authoritarian powers," they say. They are also worried about U.S. defense spending cuts and the shrinking U.S. Navy, Mitchell and Grygiel write.